Method of making a void-free aluminum film

ABSTRACT

A method for depositing an aluminum film limits the growth of voids and notches in the aluminum film and forms and aluminum film with a reduced amount of voids and notches. The first step of the method is to form an underlying layer upon which is deposited an aluminum film having a first thickness. The surface of the aluminum film is then exposed to a passivation species which coats the aluminum grains and precipitates at the grain boundaries so as to prevent grain movement. The exposure of the aluminum film to the passivation species reduces void formation and coalescence of the voids. An aluminum layer having a second thickness is then deposited over the initially deposited aluminum layer. In a second embodiment of the invention, the passivation species is deposited with MOCVD and to form an electromigration-resistant alloy. A third embodiment involves multiple depositions of aluminum, with exposure to a passivation species conducted after each deposition. Each deposition is also conducted at a successively lower temperature than the prior deposition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 09/294,771, filed on Apr. 19, 1999, now U.S. Pat. No. 6,255,212 B1, which is a divisional of U.S. patent application Ser. No. 08/977,786, filed on Nov. 25, 1997, now U.S. Pat. No. 5,963,835, which is a divisional U.S. patent application Ser. No. 08/802,405, filed on Feb. 18, 1997, now U.S. Pat. No. 5,844,318, all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to the formation of aluminum films. More particularly, the present invention relates to the formation of an aluminum film with a reduced occurrence and size of voids in a process suitable for forming contacts, vias, and metal interconnect lines situated on a semiconductor substrate assembly.

2. The Relevant Technology

Integrated circuits are manufactured by an elaborate process in which a variety of different semiconductor devices are integrally formed on a small silicon wafer. Conventional semiconductor devices include capacitors, resistors, transistors, diodes, and the like. In advanced manufacturing of integrated circuits, hundreds of thousands of semiconductor devices are formed on a single wafer.

One step in the manufacture of integrated circuits is to form metal interconnect lines between the discrete semiconductor devices on the integrated circuit. The metal interconnect lines allow for an electrical current to be delivered to and from the semiconductor devices so that the integrated circuit can perform its intended function.

The metal interconnect lines generally comprise narrow lines of aluminum etched from an aluminum film. Aluminum is typically used because it has relatively low resistivity, good current-carrying density, superior adhesion to silicon dioxide, and is available in high purity. Each of these properties is desirable in contact lines since they result in a faster and more efficient electronic circuit.

A further step in which aluminum films are used and which is frequently conducted in the manufacturing of integrated circuits is the formation of inter-level interconnect structures between nonadjacent layers on the integrated circuit wafer. These inter-level interconnect structures include contacts, plugs, and vias. An example of an interconnect structure in the form of a contact is shown in FIG. 1. In FIG. 1, a contact is shown with a contact opening 14 extending through a passivation layer 12 down to a silicon substrate assembly 10. A substrate assembly is intended herein to mean a substrate having one or more layers or structures thereon. Contact opening 14 is filled with a titanium nitride diffusion barrier 15 and a film of aluminum 16.

One complication arising from the use of aluminum films is the phenomenon referred to as “void formation.” Void formation is a process in which minute voids 18, shown in FIG. 1, form between grain boundaries as grains move and grow as a result of heat treatments following metal deposition. Void formation is believed to stem from the solid phase growth of aluminum crystals as the film becomes thicker during deposition. Crystal growth causes grain boundary movement and gives rise to voids and notches at the interface between two adjacent grains. The voids, also known as “notches,” coalesce at flux divergence sites such as grain boundary triple points of a metal interconnect line, contact, or via. Voids cause a heightened resistivity in the aluminum film, thereby reducing the effectiveness of the aluminum film in providing electrical communication.

Void formation is accompanied by a phenomenon known as electromigration. Electromigration occurs as an electrical current flows through an aluminum portion of an interconnect line, contact, via, or other structure. When a voltage is applied across the aluminum, electrons begin to flow through the aluminum. These electrons impart energy to the aluminum atoms sufficient to eject aluminum atoms from their lattice sites. As the aluminum atoms become mobile, they leave behind vacancies. In turn, the vacancies are also mobile, since they can be filled by other aluminum atoms which then open new vacancies. In the phenomenon of electromigration, the vacancies formed throughout the aluminum line tend to coalesce at the grain boundary triple points of the metal line, thereby also helping to form voids.

In a further complication, voids 18 tend to coalesce due to increased movement of the grains at higher temperatures to form large-scale voids 20, as shown in FIG. 2. Large-scale voids 20 severely limit conductivity and can cause failure of the interconnect, via, or contact. This can result in the failure of the semiconductor device as a whole and failure of the entire integrated circuit. This problem occurs more frequently and is increasingly problematic at greater miniaturization levels due to the smaller size of the structures in relation to large scale voids.

In one attempt to eliminate void formation, aluminum is mixed with other metals to form an aluminum alloy. For example, copper is frequently added to aluminum. The addition of copper increases the energy required to cause the voids to form in the metal interconnect line. Titanium and other metals are also frequently deposited together with the aluminum, which metals are then alloyed to the aluminum in a high temperature anneal process flow step. Alloying is only a partial remedy, however, as void formation still continues to occur over time, especially as the size of aluminum features decrease.

To ensure adequate coverage of aluminum films, which is desirable in filling high aspect ratio via and contact holes, and in maintaining low contact resistance and consistent etching, the prior art has turned to cold wall CVD aluminum deposition processes. One example of this is organometallic chemical vapor deposition (MOCVD) using dimethyl aluminum hydride (DMAH). When so doing, a base layer of titanium nitride is typically deposited by chemical vapor deposition prior to the deposition of the aluminum film. The base layer of titanium nitride assists in the process of achieving uniform nucleation of the aluminum film. While this process is somewhat beneficial, the deposited aluminum films continue to exhibit voids 18 in aluminum film 28 and at the aluminum-titanium nitride interface due to grain growth, an illustration of which appears in FIG. 3.

FIG. 3 shows a substrate 10 subjacent to a passivation layer 22 composed of, for example, BPSG or SiO₂. A titanium nitride base layer 26 is formed over passivation layer 22 and is subjacent an aluminum film 28. A plurality of voids 18 have formed in aluminum film 28 and at an interface 24 between titanium nitride base layer 26 and aluminum film 28. Voids 18 result in increased resistance in aluminum film 28, reliability problems of integrated circuits being formed by the structure of FIG. 3, and ultimately device failure that results in lowered yield rates of the process by which the integrated circuits are manufactured.

As a result of the above discussion, it is apparent that a need exists in the art for a method of aluminum film deposition which can be used to form uniform aluminum films with substantially eliminated or arrested void formation. Such a method would be additionally beneficial if it could be used for forming aluminum interconnect lines and inter-level interconnect structures, if it could be used with alloying processes, and if it could provide reduced electromigration.

BRIEF SUMMARY OF THE INVENTION

In accordance with the invention as embodied and broadly described herein in the preferred embodiment, a method is provided for depositing aluminum films with a reduced occurrence and size of voids and notches. The method comprises in a first step, forming an underlying layer. One preferred underlying layer is titanium nitride which is deposited in a conventional manner.

In a further step, aluminum of a predetermined first thickness is deposited over the titanium nitride underlayer. The first thickness of aluminum is typically between about one third and one half of the total desired thickness of the aluminum film being deposited. The aluminum is preferably deposited with a cold wall chemical vapor deposition process, but could be deposited using any suitable process. The surface of the first thickness of aluminum is then exposed to a passivation species which coats the aluminum grain and precipitates at the grain boundaries. The passivation species comprises compounds such as O₂, N₂, TiCl₄, TDMAT, SiH₄, GeH₄, as well as members of the halogen and interhalogen families of gases. Other species which achieves surface passivation can also be used.

In a further step, an aluminum film having a second thickness is deposited over the aluminum film having the first thickness. In the basic embodiment, the second thickness of the later deposited aluminum film comprises the remainder of the final thickness of the aluminum film desired to be formed.

The passivation species to which the surface of the aluminum film having the first thickness is exposed causes friction between the aluminum grains, both chemically and mechanically, so as to arrest and make the grains of the aluminum at the surface immobile. Immobilizing the grains causes a reduction in movement across grain boundaries and a suppression of grain growth. This in turn inhibits voids from forming and prohibits voids that do form from coalescing to form large-scale voids. The result is an aluminum film with a reduced amount of voids and notches from the prior art and with a reduced coalescence of the voids that are present.

In a second embodiment, the passivation species is deposited with CVD as organometallic chemical vapor deposition (MOCVD). The passivation species used in the second embodiment may comprise, for instance, Ti, TiN, TiC_(x)N_(y), TiC_(x)N_(y)O_(z), Cu, Sc, C, Si, Ge, and Sb. The MOCVD deposited passivation species achieves surface passivation as described above, and in addition, forms an alloy with the adjoining aluminum films, thereby improving electromigration resistance.

A third embodiment involves multiple depositions of aluminum. Under this embodiment, an aluminum film having a first thickness is deposited and then passivated in the manner discussed above. An aluminum film having a second thickness is deposited and is also passivated. Additional aluminum films are then subsequently deposited, each having a predetermined thicknesses. A layer of passivation species is deposited after each aluminum film deposition, and the process is repeated until the final desired thickness of a resultant composite aluminum film is obtained. Additionally, each subsequent deposition is preferably conducted at a successively lower temperature, further helping to suppress void formation and to reduce coalescence of voids into large-scale voids.

An aluminum film structure is produced by the method of the present invention, having a substrate assembly and a plurality of adjacent layers situated upon the substrate assembly. Each adjacent layer has a substantially uniform thickness and an upper surface. The upper surface has a plurality of aluminum grains that are substantially immobile such that the grain boundary movement thereof is substantially arrested. Each adjacent layer has substantially no voids larger than the thickness thereof.

The aluminum film produced by the method of the present invention is highly suitable for forming metal interconnect lines and for forming inter-level interconnect structures down to semiconductor devices on a substrate assembly. The aluminum film has less voids and notches than if the passivation species were not used, and any voids and notches that do exist will not cross over the boundaries between the discrete layers laid by the multiple deposition steps. This in turn limits the maximum diameter of voids that can occur to the thickness of any of the discrete layers.

The above discussion illustrates several embodiments of a method for forming aluminum films. The method is suitable for use in forming aluminum interconnect lines, contacts, vias, and for other aluminum film applications. The method provides an interface with reduced occurrence and size of voids and notches, reduced electromigration, and increased conductivity. Faster device speeds and improved yield rates are also a result of the inventive method.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1 is a cross-sectional depiction of a contact opening filled by an aluminum film on an in-process integrated circuit wafer, showing the occurrence of voids within the aluminum film of the contact opening.

FIG. 2 is a cross-sectional depiction of the contact opening of FIG. 1 after higher temperature processes have been conducted and showing the coalescence of the voids into a larger void.

FIG. 3 is a cross-sectional depiction of a film of aluminum deposited under a method of the prior art and showing voids at the interface between aluminum and titanium nitride layers.

FIG. 4 is a cross-sectional depiction of an aluminum film deposited under a basic embodiment of the method of the present invention.

FIG. 5 is a cross-sectional depiction of aluminum film deposited under a second embodiment of the method of the present invention.

FIG. 6 is a cross-sectional depiction of a contact opening filled with an aluminum film in accordance with the second embodiment of the method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method of the present invention will be described in several embodiments illustrated in FIGS. 4 through 6 of the accompanying drawings. The first step of the present invention is illustrated in FIG. 4 and comprises the deposition of an underlying layer 30 on a substrate assembly 10. Substrate assembly 10 typically comprises a substrate upon which may be formed one or more structural layers. In the depicted embodiment, substrate assembly 10 comprises a silicon substrate of an in-process integrated circuit wafer. Underlying layer 30 preferably comprises titanium nitride, and in the depicted embodiment, is formed over a passivation layer 22 of BPSG or SiO₂.

When the aluminum film is being used to form a contact or via, as in the embodiment of FIG. 6, a contact opening 52 is first formed through passivation layer 22 down to an underlying active region, semiconductor device, or other such surface which otherwise must be electrically interconnected with a surface layer. In such an application, underlying layer 30 is then deposited in contact opening 52 to serve as a diffusion barrier.

Returning to the planar film embodiment of FIG. 4 for discussion purposes, after underlying layer 30 is deposited, an aluminum film 32 having an initial thickness of a fraction of the final desired thickness of aluminum film 32 is deposited over underlying layer 30. Aluminum film deposition is preferably conducted with a cold wall CVD process such as that described above, though it should be apparent to one skilled in the art that any suitable deposition process could be used. The thickness of aluminum layer 32 is preferably about one third to one half of the final thickness of the aluminum film which is desired to be deposited.

In a further step, the surface of aluminum film 32 is exposed to a species which achieves surface passivation. The passivation species is preferably deposited as a passivation layer 36 which coats at least the top surface of aluminum film 32. Passivation layer 36 can also be allowed to penetrate into aluminum film 32 to pacify portions of aluminum film 32 beyond the top surface.

Examples of suitable passivation species comprise O₂, N₂, TiCl₄, tetradimethylaminotitanium (TDMAT) SiH₄, GeH₄, along with members of the halogen and interhalogen families of gases. The passivation species is deposited in such a manner as to at least partially coat the aluminum grains. This will allow the passivation species to precipitate in the grain boundaries, and thereby cause friction and the freezing in place of the grains of aluminum film 32. Further grain growth during subsequent aluminum film deposition is arrested as a result. Arresting the movement of the grains of aluminum film 32 substantially reduces the formation of voids as well as the coalescence of voids into large-scale voids.

An aluminum film 34 having a second thickness is then deposited over aluminum film 32. The second thickness of aluminum film 34, in the basic embodiment of the invention, makes up the balance of the final thickness of the aluminum film which is desired to be deposited.

Alternative embodiments are also provided in the present invention. In one alternative embodiment, the passivation species comprises a material such as Ti, Cu, C, TiN, TiC_(x)N_(y), TiC_(x)N_(y)O_(z), Si, Ge, Sc, and Sb, etc. which combines and alloys with the aluminum to not only passivate the surface, but also to reduce electromigration. The passivation species in this embodiment is preferably deposited with an organometallic chemical vapor deposition (MOCVD) process. Thus, under this embodiment, underlying layer 30 and aluminum film 32 are deposited as discussed above in reference to the embodiment of FIG. 4, and passivation layer 36, selected from the aforementioned alternate group of materials, is deposited with MOCVD. As a consequence, when depositing second thickness of aluminum 34, the selected passivation species will alloy into second thickness of aluminum, and to a lesser extent, into initial aluminum film 32, thereby causing increased electromigration resistance in both films.

In a further alternative embodiment of the method of the present invention, the deposition of aluminum is conducted in multiple steps. The initial thickness of aluminum is deposited at a higher temperature, and each deposition step is conducted at a successively lower temperature. This causes a reduction in size of the grains of the aluminum film, thereby reducing voiding and film stress and preventing grain growth in the portions of the aluminum film which have been deposited during prior steps.

This embodiment is illustrated in FIG. 5. As seen in FIG. 5 therein, an initial aluminum film 38 is deposited over underlying layer 30. A passivation species in the form of passivation layer 40 is then deposited over initial aluminum film 38 in order to suppress the formation and coalescence of voids. Subsequently thereafter, successive aluminum films represented in FIG. 5 as aluminum films 42, 46, and 50 are deposited with intervening passivation layers 44 and 48. The passivation species used to form passivation layers 40, 44, and 48 can be the same or may vary according to the inventive method.

In one example, given by way of illustration and not intended to be restrictive, initial aluminum film 38 is deposited with a thickness of about 1000 angstroms at a temperature of about 250° C. Successive aluminum films 42, 46, and 50 are deposited with a thickness of about 200 angstroms at a temperature of about 150° C. Successive depositions are conducted with intervening passivation layer deposition until the accumulated thickness of aluminum reaches about 3000 angstroms.

FIG. 6 illustrates a further embodiment of the method of the present invention. In the embodiment of FIG. 6, an inter-level interconnect structure in the form of a contact opening 52 is metallized with an aluminum film formed under the method of the present invention. Contact opening 52 electrically interconnects an active device 10 a, which in the depicted embodiment comprises a junction of a MOS semiconductor transistor. Under this embodiment, contact opening 52 is metallized in multiple steps in accordance with the discussion of FIG. 5, wherein aluminum films 38, 42, 46, and 50 are deposited with the successive intervening deposition of surface passivation layers 40, 44, and 48 to result in the inter-level interconnect structure of FIG. 6.

The size of voids that can occur in the resulting contact structure of FIG. 6 are limited to the thicknesses of each of aluminum films 38, 42, 46, and 50. That is, passivation layers 40, 44, and 48 will prohibit coalescence of voids that intersect the boundaries between aluminum films 38, 42, 46, and 50. This in turn limits the maximum diameter of the voids that can occur in contact opening 52 to a diameter of the same dimension as the thickness of any one of aluminum films 38, 42, 46, and 50. Such is also the case when planar films are formed as in the structures of FIGS. 4 and 5. Thus, if each of aluminum films 38, 42, 46, and 50 are of uniform thickness and together fill contact opening 52, substantially no voids will result that have a diameter exceeding the diameter of contact opening 52 divided by the number of aluminum films, in this case, four.

The above discussion illustrates several embodiments of a method for forming an aluminum film. The aluminum film is suitable for forming inter-level interconnect structures such as aluminum interconnect lines, contacts, vias, as well as other aluminum film structures where a need exists for a reduced occurrence of voids and notches, reduced electromigration, and increased conductivity. The inventive method has been found to promote faster device speeds as well as improved yield rates as compared to conventional aluminum film deposition.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrated and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 

What is claimed is:
 1. A method of forming an aluminum film on a substrate assembly, the method comprising: forming an underlying layer over a semiconductor layer situated on a substrate assembly; depositing by a cold wall chemical vapor deposition process a first layer of aluminum over the underlying layer; depositing by an MOCVD process on the first layer of aluminum a species to achieve a surface passivation on a top surface of the first layer of aluminum and to form an alloy on the top surface of the first layer of aluminum, the alloy having an electromigration resistance greater than that of the first layer of aluminum, and forming a second layer of aluminum over the first layer of aluminum.
 2. A method as recited in claim 1, wherein said species alloys into aluminum in said second layer to a lesser extent than said species alloys into aluminum in said first layer.
 3. A method of forming an aluminum film on a substrate assembly, the method comprising: forming a titanium nitride layer over a semiconductor substrate of a substrate assembly; depositing a first layer of aluminum over the titanium nitride layer with a cold wall chemical vapor deposition process; depositing on the first layer of aluminum a first species to achieve passivation of a top surface of the first layer of aluminum and immobilize a plurality of aluminum grains at the top surface of the first layer of aluminum; depositing with a cold wall chemical vapor deposition process a second layer of aluminum over the first layer of aluminum, wherein the aluminum of the second layer of aluminum is deposited in multiple aluminum deposition steps, each said aluminum deposition step of the multiple aluminum deposition steps depositing a plurality of aluminum layers 2 through n, each of said layers 2 through n, having an upper surface, the first aluminum deposition step of the multiple aluminum deposition steps being conducted at a lower temperature than the temperature at which the first layer of aluminum has been deposited, and each subsequent aluminum deposition step of the multiple aluminum deposition steps being conducted at a lower temperature than the immediately preceding aluminum deposition step of the multiple aluminum deposition steps; and depositing upon the upper surface of the aluminum layer deposited by each of the multiple aluminum deposition steps a plurality of species 2 through n, each of said plurality of species corresponding to one of said aluminum layers 2 through n, to achieve passivation of the upper surface thereof, and to immobilize a plurality of aluminum grains at the upper surface thereof.
 4. A method as recited in claim 3, wherein at least one of said species is deposited in an MOCVD process so as to form an alloy having an electromigration resistance greater than that of aluminum, and said at least one of said species is one of Ti, Cu, TiN, TiC_(x)N_(y), TiC_(x)N_(y)O_(z), Sc, C, Si, Ge, and Sb.
 5. A method as recited in claim 3, wherein all n said species are the same.
 6. A method as recited in claim 3, wherein said first through n species comprise a plurality of different species.
 7. A method of forming an aluminum film on a substrate assembly, the method comprising: forming a first layer of aluminum on a substrate assembly; exposing the first layer of aluminum to a species to achieve a surface passivation on a top surface of the first layer of aluminum; and forming a second layer of aluminum over the first layer of aluminum comprising multiple aluminum deposition steps, each aluminum deposition step of the multiple aluminum deposition steps comprising a deposition of an aluminum layer having an upper surface, wherein the formation of the first layer of aluminum and the multiple aluminum deposition steps are conducted at different temperatures.
 8. A method as recited in claim 7, further comprising the formation of at least one passivation layer between two of said aluminum layers having voids intersecting the boundaries therebetween, wherein said at least on passivation layer prevents the coalescence of voids that intersect the boundaries between said two of said aluminum layers.
 9. A method as recited in claim 7, further comprising the formation of at least one passivation layer between two of said aluminum layers, wherein said passivation layer comprises a species that alloys with aluminum and said alloy has an electromigration resistance greater than that of aluminum.
 10. A method of forming an aluminum film on a substrate assembly, the method comprising: forming a first layer of aluminum on a substrate assembly; exposing the first layer of aluminum to a species to achieve a surface passivation on a top surface of the first layer of aluminum; and forming a second layer of aluminum over the first layer of aluminum comprising multiple aluminum deposition steps, each aluminum deposition step of the multiple aluminum deposition steps comprising a deposition of an aluminum layer having an upper surface, wherein the formation of the first layer of aluminum and each of the aluminum deposition steps are conducted at different temperatures, and wherein at least one aluminum layer that is deposited during the multiple aluminum deposition steps is exposed to a species in order to achieve surface passivation of the upper surface of the at least one aluminum layer such that a plurality of aluminum grains at the upper surface are immobilized, the species being deposited by an MOCVD process.
 11. A method of forming an aluminum film on a substrate assembly, the method comprising: forming a first layer of aluminum on a substrate assembly; exposing the first layer of aluminum to a species to achieve a surface passivation on a top surface of the first layer of aluminum; and forming a second layer of aluminum over the first layer of aluminum comprising multiple aluminum deposition steps, each aluminum deposition step of the multiple deposition steps comprising a deposition of an aluminum layer having a grain size and an upper surface, where in the grain size of each of said aluminum layers in smaller than the grain size of the preceding aluminum layer, and wherein at least one aluminum layer that is deposited during the multiple aluminum deposition steps is exposed to a species in order to achieve surface passivation of the upper surface of the at least one aluminum layer such that a plurality of aluminum grains at the upper surface are immobilized, the species being deposited by an MOCVD process.
 12. A method as recited in claim 11, wherein said species is one of Ti, Cu, TiN, TiC_(x)N_(y), TiC_(x)N_(y)O_(z), Sc, C, Si, Ge, and Sb.
 13. A method of forming an aluminum film on a substrate assembly, the method comprising: forming an underlying layer over a semiconductor layer situated on a substrate assembly; depositing by a cold wall chemical vapor deposition process a first layer of aluminum over the underlying layer; depositing by an MOCVD process on the first layer of aluminum a species to passivate the first layer of aluminum and to form at least on a top surface of the first layer of aluminum, a composition having an electromigration resistance greater than that of the first layer of aluminum; and forming a second layer of aluminum over the first layer of aluminum comprising multiple aluminum deposition steps, each aluminum deposition step of the multiple aluminum deposition steps comprising a deposition of a plurality of aluminum layers 2 through n having an upper surface; wherein n labels the last of said deposited aluminum layers.
 14. A method as recited in claim 13, wherein the first aluminum deposition step of the multiple aluminum deposition steps is conducted at a lower temperature than the temperature at which the first layer of aluminum has been deposited, and each subsequent aluminum deposition step of the multiple aluminum deposition steps is conducted at a lower temperature than the immediately preceding aluminum deposition step of the multiple aluminum deposition steps.
 15. A method as recited in claim 14, wherein at least one aluminum layer that is deposited during the multiple aluminum deposition steps is exposed to a species to passivate at least the upper surface of the at least one aluminum layer such that a plurality of aluminum grains at the upper surface are immobilized, the species being deposited by an MOCVD process.
 16. A method as recited in claim 13, wherein the first layer of aluminum has a first grain size and each of said 2 through n aluminum layers deposited in each of said steps has a grain size 2 through n respectively, and wherein at least two of said first and said 2 through n grain sizes are different.
 17. A method as recited in claim 14, wherein said first and said 2 through n grain sizes progressively decrease such that the first grain size is the largest and the n grain size is the smallest.
 18. A method of forming an aluminum film, the method comprising: depositing by a cold wall chemical vapor deposition process a first layer of aluminum over a substrate; depositing by an MOCVD process on the first layer of aluminum a species to passivate said first layer of aluminum and to increase its electromigration resistance, wherein the species is one of Ti, Cu, TiN, TiC_(x)N_(y), TiC_(x)N_(y)O_(z), Sc, C, Si, Ge, and Sb; and forming a second layer of aluminum over the first layer of aluminum.
 19. A method of forming an aluminum film, the method comprising: depositing a first layer of aluminum over a refractory metal nitride layer with a cold wall chemical vapor deposition process; immobilizing a plurality of aluminum grains on a top surface of the first layer of aluminum; forming over the first layer of aluminum a plurality of aluminum layers each: being formed by cold wall chemical vapor deposition; comprising aluminum grains smaller than the aluminum grains of the immediately preceding aluminum layer; and having a plurality of aluminum grains upon an upper surface thereof that are immobilized; and depositing a final layer of aluminum over the upper surface of the aluminum layer deposited by the last cold wall chemical vapor deposition. 